1. Field of the Invention:
The invention relates to shaped articles formed from glass fiber reinforced gypsum and more particularly to a process for making such shaped articles.
2. Description of the Prior Art:
Gypsum has been used as a casting and molding material for many years. Gypsum is known as hydrated plaster of Paris which is the hemihydrate of calcium sulfate. One hundred parts by weight of the calcium sulfate hemihydrate combined stoichiometrically with 18.6 parts by weight of water to form a hard, set plaster containing two mols of combined water. In order to prepare a workable, pumpable, moldable compostion, the calcium sulfate hemihydrate is combined with an excess of water in addition to the 18.6 parts by weight which are required for the conversion of the hemihydrate into a set plaster. With ordinary calcined calcium sulfate hemihydrate, also known as beta hemihydrate, the calcium sulfate hemihydrate is combined with more than 50 percent of its weight of water in order to achieve a pouring consistency. It is possible to achieve a pouring consistency with less than 50 percent water when the calcium sulfate hemihydrate is in the form of crystalline calcined calcium sulfate hemihydrate also known as alpha hemihydrate. See U.S. Pat. No. 1,901,051--RANDEL et al. Moldable compositions containing 40 parts of water for every 100 parts of dry powder (predominantly alpha hemihydrate) have been described. See U.S. Pat. No. 2,494,403--NIES et al.
Structurally reinforced articles formed from gypsum and glass fibers have been described wherein the glass fibers are mixed into a slurry of calcium sulfate hemihydrate and water. See U.S. Pat. No. 3,062,670--MARZOCCHI et al; U.S. Pat. No. 2,681,863--CROCE et al and U.S. Pat. No. 3,147,127--SHANNON. In all of these glass fiber reinforcement processes involving preparation of a slurry containing glass fibers, the act of mixing the fibers introduces a tendency to break the fibers into short lengths. It has been reported that 0.1 percent of textile fibers (diameter 0.0004 inch) cannot be admixed with the calcium sulfate hemihydrate slurry whereas 3 percent of larger diameter fibers (0.003 inch) can be added to a moldable calcium sulfate hemihydrate slurry with ease. See U.S. Pat. No. 3,062,670 supra.
A significant development in glass fiber reinforced gypsum technology is set forth in British Pat. No. 1,204,541--National Research Development Corporation. The significant new development avoids mixing of glass fibers in a calcium sulfate hemihydrate slurry but instead prepares an admixture of calcium sulfate hemihydrate, water and glass fibers by spraying an aqueous slurry of the calcium sulfate hemihydrate into a stream of freshly chopped glass fibers or onto a performed mat formed from randomly oriented glass fibers. Glass fiber reinforced gypsum is known as GRG.
In order to achieve adequate wetting of the glass fibers in the British process, a substantial excess of water is employed in the aqueous slurry--that is an excess over the stoichiometric amount required to combine with the calcium sulfate hemihydrate. Slurries containing 50 parts by weight of water and 100 parts by weight calcium sulfate hemihydrate are contemplated. Water-to-calcium sulfate hemihydrate weight ratios of 0.4 to 0.6 are described, J. Materials Science, Vol. 4(5), May 1969, pp. 389-395. The British patent process thus prepares a watery slurry containing calcium sulfate hemihydrate and glass fibers. An essential feature of the British process is the deliberate removal of excess water prior to the setting of the plaster mixture. The excess water is initially removed by vacuum removal or by pressure to produce a composition which still contains an excess of water over the stoichiometric amount required for the calcium sulfate hemihydrate and contains enough water to provide a moldable and workable plaster which exists for a short period of time until the gypsum becomes set. The removal of the excess water is a difficult task. One technique for removing the water has been to form the dilute slurry on a porous membrane, such as a sheet of Kraft paper, and to pass the porous membrane containing the dilute slurry over a suction box which has facilities for extracting water from the dilute slurry through the pores of the Kraft paper. Nonetheless, the British patent process is capable of producing glass fiber reinforced gypsum articles of remarkable strength characteristics as a result of retaining relatively long length glass fibers in a random orientation in the final article.
It would certainly be desirable to eliminate the cumbersome and expensive water removal stage which is necessitated in the process described in the British patent. It is also desirable to develop a process for producing glass fiber reinforced gypsum articles on a continuous basis in a variety of profiled shapes. Such profiled shapes can be employed in producing products of the type described in U.S. Pat. Nos. 3,842,559; 3,839,836, which are assigned to an assignee of the present invention. The profiled shapes also can be employed to produce liner sheets for building construction panels as will be hereinafter described.